The invention relates generally to safety and arming devices used in demolition systems, and more particularly to a safety and arming device for use in a water environment that utilizes an integrated water sensor/actuator using a cellulose-based moisture absorbing material.
Some military explosive systems used in maritime environments are required to first sense the presence of water and then, only after water is sensed, actuate the elements of a device""s operational safety and reliability sequence. That is, the system must be xe2x80x9csafedxe2x80x9d in air and armed only after entering a water environment. For example, a fuze on an air-launched projectile/weapon typically uses a sensor to sense the presence of water and an actuator to initiate an arming sequence. Usually, the sensing and actuation functions are achieved by two separate devices within the fuze where actuation of critical logic gates (e.g., mechanical, electrical or chemical gates) depends on a signal from the water sensing portion of the fuze. Since standards governing premature actuation (i.e., prior to water being sensed) generally dictate a failure rate of less than one failure in a million, it is imperative that the two separate devices perform reliably both individually and in combination with one another. However, such coordinated operation typically utilizes a complex and expensive mechanism that is inherently prone to failure owing to its complexity.
In an attempt to simplify the sensing/actuation problem, the water sensing and actuation functions could be integrated with one another. U.S. Pat. No. 6,182,507 describes one such prior art integrated mechanical water sensor in which compressed cotton balls are constrained in an open frame as a means to provide for water absorption and subsequent cotton expansion where the force of expansion is used to move a piston. However, compressed cotton balls do not provide a reliable means of moisture absorption in harsh underwater environments and, therefore, are not reliable as a means of producing work when subjected to immersion in such environments. This is because the compressed cotton balls rely on surface absorption of moisture for its expansion. However, high-levels of naturally-occurring impurities and man-made pollutants often found in underwater environments can cover the surface area of the cotton thereby impeding the absorption of water.
Accordingly, it is an object of the present invention to provide a safety and arming device that is xe2x80x9csafedxe2x80x9d in air and armed only after entering a water environment.
Another object of the present invention is to provide a safety and arming device that can function in moisture environments having impurities.
Yet another object of the present invention is to provide a safety and arming device that integrates it""s sensing and actuating functions with a single structure.
Still another object of the present invention is to provide an safety and arming device that functions reliably in harsh underwater environments.
Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings.
In accordance with the present invention, a safety and arming device has first and second portions of an explosive train fixably supported in a housing such that they are operably separated from one another. A third portion of the explosive train is movably mounted in the housing. More specifically, the third portion is fixable in a first position such that the first and second portions remain operably separated from one another. However, the third portion is movable to a second position such that the first and second portions are operably coupled to one another via the third portion. An integrated water sensor/actuator is coupled to the third portion to provide the motive force that moves the third portion to its second position. The water sensor/actuator is based on a fibrous cellulosic material having anisotropic moisture-absorbing properties such that dried-in strain of the cellulosic material is greatest along one axis thereof. In the invention, a plug of the dry and compressed fibrous cellulosic material has a powder material coated thereon and mixed therewith. The plug is compressed along its axis of greatest dried-in strain and is fitted in a portion of a water-permeable housing adjacent one end thereof. The powder material is inert with respect to the cellulosic material and initiates a chemical reaction when exposed to water such that a product of the chemical reaction is water. A piston is fitted in the housing adjacent the plug. Exposure of the plug to water causes its expansion and corresponding movement of the piston which, in turn, moves the third portion to the second position.